One such tension measuring device is described in French patent application No. 89 16 189 filed on Dec. 7, 1989 in the name of GEC ALSTHOM SA.
FIG. 1 accompanying the present specification shows the electrical circuit used to obtain an image of the tension that is to be measured.
The line L1 designates a high tension conductor whose tension Up is to be measured.
The circuit comprises a first capacitor C1 referred to as the high tension capacitor that is connected via a link cable L2 to the negative input of an operational amplifier A which is provided with a second capacitor C2 connected between its output and its negative input. The positive terminal of the operational amplifier is connected to ground. The capacitor .gamma. represents stray capacitance taken as a whole and including, in particular, the capacitance of the link cable L2 and the capacitance formed by the low tension capacitor plate and ground. The capacitance of the cable L2 is not negligible since the operational amplifier A is situated in a measurement room B that may be several tens of meters distant from the location in which the high tension capacitor C1 is placed.
The output voltage Us of the operational amplifier is an image of the voltage to be measured since these two magnitudes are related by the equation: EQU Up=-Us.multidot.C2/C1
In order to ensure that the image Us is independent of temperature, it is necessary for the ratio C2/C1 to be independent of temperature.
The capacitor C2 is located in the measurement room and it is easy to place it in a temperature-regulated enclosure.
However the capacitor C1 is located in the apparatus or in the vicinity thereof, and it often happens that the apparatus is out-of-doors where temperature may vary over a very wide range, e.g. -50.degree. C. to +60.degree. C.
Measurement accuracy therefore depends on the variation in the capacitance of the capacitor C2 with temperature. To obtain an accurate measurement of the tension Us regardless of the temperature to which the capacitor C1 is subjected, it is necessary for the capacitance of said capacitors to be independent of temperature.